Prior art attempts at reducing the NO.sub.x component present in flue gases, a proven contributor to environmental pollution, have involved returning a portion of the flue gas back to the combustion chamber, which is then mixed with the combustion air. In such conventional systems, a well-insulated return duct, fitted to the outside of the boiler, is connected to the flue gas-collecting region of the boiler to the intake region of the impeller housing of the burner. In German Utility Patent No. U-87 08 656, the requirement for a long flue gas return duct is eliminated by the direct mounting of the burner, and in this case where there is a gravity-type burner, on the upper portion of the boiler casing. This system, however, uses a relatively short duct to return flue gas from a flue gas collection chamber, which is located inside the boiler, back to the impeller chamber of the burner, where such flue gases are mixed with the incoming combustion air.
Common to all of the prior art is the provision both of flue gas return ducts of varying lengths and of impeller units aiding the return of flue gas to the burner. Each of the extra impeller units is provided with its own power source. A third common factor is the direct return of flue gases through the burner's air intake or air mixture path, an arrangement that allows the burner, which is provided with an injection nozzle, to be charged with flue gases. Disclosed in German Utility Patent No. U-88 12 090 is a system designed to prevent the burner itself from being charged with gas. In this embodiment, the exhaust section of the flue gas return duct is aimed at the flame region of the burner. This design basically employs ducting and support apparatus located on the outside of the boiler casing and burner, rather than a direct connection between boiler and burner.